An oxygen oxidation reaction which oxidizes a hydrocarbon with oxygen or air in the presence of an oxidation catalyst plays a highly important role in the organic chemical industry. Examples of an oxygen-containing organic compound obtained by such a reaction include ketones such as acetone, cyclohexanone, and cyclopentanone; carboxylic acids such as terephthalic acid, phthalic anhydride, and maleic anhydride; and alkylene oxides such as ethylene oxide.
The oxygen-containing organic compounds include aromatic alcohols, many of which are important as a basic chemical product used in the organic chemical industry. Of these, phenol and cresol are particularly important chemical products. Phenol or cresol is subjected to a polycondensation reaction with formaldehyde to produce a phenol resin or cresol resin, for example. These resins are broadly used as a coating material, lacquer, or resin raw material for compression molding or foam molding. Phenol is used as a raw material of bisphenol A or bisphenol F which is important as a raw material of an epoxy resin. Cyclohexanol obtained by hydrogenating phenol is used for producing ε-caprolactam which is a raw material for nylon.
Roughly two methods for producing phenol which is important among aromatic alcohols are known in the art. In one method, benzene or alkylbenzene is chemically oxidized to synthesize phenol. In the other method, tar which is obtained by dry distillation of coal is fractionated or extracted to produce phenol. Phenol produced utilizing the latter method has a low purity due to many impurities. Accordingly, an indirect method which oxidizes alkylbenzene is mainly utilized at present.
As a method for industrial production of phenol based on the above oxidation method, a “direct oxidation method” which partially oxidizes benzene directly is the most ideal. Since it is difficult to control the reaction when the “direct oxidation method” is utilized, phenol produced by oxidizing benzene is further oxidized. Therefore, the method has not been practical until now.
Of the indirect methods for producing phenol, a cumene method is the most popular. The cumene method comprises synthesizing cumene by reacting propylene with benzene, oxidizing cumene with air using a cobalt salt catalyst, for example, to produce cumene hydroperoxide, and decomposing the resulting cumene hydroperoxide into phenol and acetone by the action of an acid catalyst. This method is highly excellent due to a high selectivity of phenol. However, acetone is also produced with the same molar ratio as phenol. Therefore, the cost of phenol fluctuates in accordance with the acetone demand.
Some “direct oxidation methods” have recently been proposed. For example, a paper of G. I. Panov (see Appl. Catal. A., 98, 33 (1993)) discloses a method for directly oxidizing benzene using nitrous oxide as an oxidizer to obtain phenol. A problem with this method is in the difficulty of synthesizing nitrous oxide. Japanese Patent Applications Laid-open No. 6-1738 and No. 7-69950 propose a method for oxidizing benzene using hydrogen peroxide as an oxygen source in the presence of various catalysts such as iron or a noble metal supported on a carrier, zeolite, and heteropolyacid. This method is environmentally friendly since only water is the bi-product. However, a huge amount of expensive hydrogen peroxide is needed. A paper of Yamanaka, Otsuka, et al. (see Appl. Catal. A., 171, 309 (1998)) discloses that benzene is oxidized using oxygen gas in the presence of an europium catalyst carried on titania. In this method, the manner of handling the europium catalyst is complicated, and the yield of phenol is only 2-4%.
Thus, since no economically satisfactory process for producing phenol by directly oxidizing benzene has been developed, phenol is usually produced by the indirect oxidation method. Similar circumstances can be found in the case of producing propylene oxide by oxidizing propylene.
However, the indirect oxidation method involves many complicated reaction steps and production of unnecessary bi-products. Therefore, development of the direct oxidation method for directly oxidizing a hydrocarbon has been desired.
Conventionally, as the direct oxidation method for producing an oxygen-containing organic compound, a method comprising first mixing a hydrocarbon as a raw material with a gas such as oxygen and then circulating the mixture in a fixed bed circulation reaction apparatus filled with a solid catalyst has been known. This method, however, has a problem of an extremely low reaction yield.
One reason for the low reaction yield is the low selectivity of the target compound. Since the oxygen-containing compound produced by the reaction has a decreased molecular ionization potential, the compound is oxidized more easily than the raw material hydrocarbon. Therefore, the target product is successively overreacted (oxidized), which leads to a decreased selectivity of the product. In order to control this overreaction, reaction conditions in which the concentration of raw materials greatly exceeds the concentration of the product must be employed. This type of reaction in which a flammable material is reacted with oxygen inducing combustion (a combustion promoter) involves an explosion risk. To avoid the explosion risk, the product cannot but be produced at a low reaction yield under low concentration conditions. These are the reasons for the low reaction yield.
Published Japanese Translation of PCT Publication for Patent Application No. 11-510817 discloses a gaseous phase oxidation reaction of propylene into propylene oxide in the presence of a silver catalyst carried on a solid carrier, for example. According to examples specifically disclosed, the raw material propylene concentration in a gas mixture introduced into a reactor is as low as 10% or less, and the conversion rate of propylene is as low as 3-5%.
As described above, in a system in which oxygen, a raw material hydrocarbon, and a product coexist, it is essentially difficult to produce the target product at a high yield while avoiding an explosion risk and preventing successive oxidation reactions.
The use of a diaphragm type reactor called a membrane reactor in a gaseous phase oxidation reaction using a hydrocarbon as a raw material has been reported. For example, Japanese Patent Application Laid-open No. 5-238961 discloses that the membrane reactor can be utilized to produce C2 hydrocarbons based on an oxidative coupling reaction of methane.
The diaphragm type catalyst used in this Patent Application is a complex oxide having a high oxygen ionic mobility and mixing conductivity. This is an ion conductor involved in the reaction which converts oxygen introduced from one side of the diaphragm into oxygen ion O2− to circulate and discharges the oxygen ion to the other side of the diaphragm. However, since oxygen moves or is supplied slowly in the ion conductor, it is difficult to achieve a reaction speed applicable for the actual production of chemicals in the organic chemical industry.
Japanese Patent Application Laid-open No. 5-194281 discloses a method for a catalytic dehydration reaction of a saturated hydrocarbon using a hydrogen permeation membrane and a dehydration catalyst in combination. Utilizing this method, hydrogen produced by a dehydration reaction permeates through the membrane to be discharged from the reaction system. As a result, the chemical equilibrium in the system is shifted to the dehydration reaction side, whereby a conversion rate above an equilibrium conversion rate is obtained.
The products obtained by the above methods using the diaphragm type reactor are hydrocarbon compounds, not oxygen-containing organic compounds. Specifically, a method for producing an oxygen-containing organic compound using the diaphragm type reactor has not been proposed so far.
Therefore, an object of the present invention to provide a method for reacting a substance to be activated by the action of a catalyst such as oxygen or hydrogen with a substance to be reacted with the activated substance such as a hydrocarbon to obtain a product at a high yield while avoiding an explosion risk and ensuring safety.